The rotor of a known pressure wave supercharger of this type, such as is described for example in the Swiss Pat. No. 633,619, is driven at a constant transmission ratio by the internal combuation engine to be supercharged via a belt mentioned and belt pulley, which is connected to the rotor shaft so as to be rotationally stiff. The rotor speed is, therefore, proportional to the engine speed and, for this reason, the expression "proportional drive" is used in this connection. Since the important feature in the interaction between a supercharging device and an angine is that it should operate with the best possible efficiency in the speed range mainly used in practical operation, the geometric data of the pressure wave supercharger control elements which are critical to the supercharger efficiency, essentially the opening and closing edges of the air and gas ports and the auxiliary ducts (the gas and compression pockets, inter alia), are designed for this speed range, which corresponds approximately to 50% of the nominal rotational speed.
This pressure wave supercharger designed for a preferred, and in fact for the operationally and economically most important, engine speed range does, however, have the disadvantage that the pressure wave process does not take place in an optimum manner in the lower and higher engine speed ranges. In these ranges, in fact, the best possible exchange of energy between the exhaust gas and charge air requires a different geometric design of the air, gas and auxiliary ports, in particular their opening and closing edges.
On the other hand, undesirable pulsations in the charge air flow, an excessive exhaust gas recirculation in the charge air, a sluggish response behavior of the rotor and a loss of efficiency appear, particularly in the lower speed range. The loss of efficiency also applies to the speed range above the design speed.
In order to avoid these disadvantages, the applicant's Swiss application No. 826/86-9 describes a free-running pressure wave supercharger driven by the gas forces. In contrast to proportional drive, the rotor speed in this concept does not depend on the engine speed but on the resultant swirl energy of all the air and gas flows acting on the rotor. By various design measures on the air, gas and auxiliary ports--in association with nozzles which come into effect under certain operating conditions--it is intended that a narrower pressure wave supercharger speed range than in the case of proportional drive shall be maintained. In particular, the measures proposed there are intended to increase the drive momentum of the exhaust gases in order to speed up the rotor after the engine has been started, to control the speed characteristic of the rotor and to prevent excessive speeds.
Satisfactory operation of this concept, however, assumes the smallest possible rotor mass moment of inertia, which affects the transient behaviour of the supercharger. If the mass moment of inertia is too large, the rotor cannot in fact follow rapid changes in the speed of the vehicle engine with sufficient rapidity so that there is a certain supercharger response delay. The conventionally used material of relatively high specific gravity is responsible for the relatively large mass moment of inertia of the conventional rotors.
It will be possible to avoid this disadvantage as soon as tested materials of lighter specific weight are available; these have to be suitable for the production without difficulty of thin-walled rotors, which have to be manufactured with great precision, and must also be capable of dealing with the conditions in a pressure wave supercharger with respect to their other thermal and mechanical properties.